THE PHYSICAL BASIS OF HEREDITY. 161 
cells are the remnants of the ends of the chromosomes 
which have been cast off in the division. In Fig. 6, D, 
the four-celled stage is shown with the karyokinetic 
figures of the next division. In the lower cells the 
spindles are seen from the pole, the chromatin is pres- 
ent in the reduced amount in the form of small granules. 
In the upper left-hand cell the two full chromosomes 
are seen, each split longitudinally, while the upper right- 
hand cell shows a repetition of the reduction phenomenon 
—viz., the central portion of the two chromosomes, 
broken up into granules, alone enters into the spindle 
figure, the outer ends being cast off into the cytoplasm, 
where they suffer a similar fate to those of the lower 
cell in the previous division. The next division repeats 
the process, one cell retaining two full chromosomes, 
while all the others have the reduced amount. This 
takes place for five successive divisions and then ceases; 
from the one cell having the two full chromosomes, 
the reproductive tissues develop, the others with reduced 
chromatin form the somatic tissues. Thus is accom- 
plished a wéstble structural differentiation of the nuclei of 
the reproductive cells which distinguishes them sharply 
from all the somatic tissues in Ascarzs, We shall see 
further on that there is abundant evidence in favour of 
the theory that the nucleus—i. e., the chromatin—is the 
bearer of hereditary influences from one generation to 
the next, and that the specific development and functions 
of each individual cell are dependent upon the specific 
changes which take place in the chromatin of its nucleus. 
In this light the almost isolated case of Ascaris pos- 
sesses a value and interest that can not be overesti- 
mated. 
While in the higher forms of animals and plants we 
find a sharp differentiation of their tissues into somatic 
and reproductive or germ cells, we must bear in mind 
